Project Summary/Abstract There is an ongoing debate about the right amount of human sun exposure in order to maximize the health bene- ?ts arising from the photosynthesis of vitamin D in epidermal skin cells. The speci?c conditions of skin exposure to ultraviolet radiation lead to varying amounts of produced vitamin D photo isomers. Wavelength, intensity, and duration of light exposure, as well as skin pigmentation or the presence of sunscreen have signi?cant in?uence on the composition and amount of vitamin D photo isomers produced by irradiation. In order to give recommen- dations of healthy sun exposure, public health policy is in dire need of a detailed understanding of the underlying molecular mechanisms. In particular, it is necessary to quantitatively understand the relationship between irradia- tion conditions and production of vitamin D photoisomers. In applying non-adiabatic ab initio molecular dynamics simulations combined with experimental femtosecond-resolved spectroscopic measurements to investigate the photo dynamics of vitamin D isomers in phospholipid bilayers, this proposal seeks to unravel the mechanisms of vitamin D photosynthesis at physiological conditions. By explicitly including the chemical environment of the membrane, our approach goes signi?cantly beyond simpli?ed gas phase and solution phase studies and can be directly compared to vitamin D photosynthesis in vivo. This combined theoretical/experimental approach is unprecedented in vitamin D research and will lead to new insights, providing a molecular picture of the processes with femtosecond resolution in time. The results of this study will signi?cantly progress the understanding of the vitamin D photosystem on a molecular level that can be used to weigh up health bene?ts of sun exposure against its risks. Furthermore, the photochemical mechanism can be put in relation to other parts of vitamin D research in order to construct a comprehensive picture, providing a solid basis for public health recommendations.